The present invention relates to excitation systems for large fluid-cooled dynamoelectric machines and in particular to static excitation systems.
Excitation systems for large dynamoelectric machines, such as the turbine-generator combinations employed by electric utilities, have grown in power rating along with the power ratings of the generators themselves. Early excitation systems included rotating power sources such as a separate dc generator driven by the turbine-generator shaft which supplies excitation current to the rotating field windings through slip rings and brushes. Another approach has employed an ac exciter driven by the turbine-generator with rectification and control of the excitation voltage in external stationary rectifier banks. Still another approach has employed the use of diode rectifiers on the rotor. In these arrangements, the whole rectification power source rotates, with control effected through electromagnetic flux linkages with the rotating components. An example of an excitation system having rotating rectification means is found in U.S. Pat. No. 3,768,502 issued Oct. 23, 1973 to Drexler et al.
A separate category of excitation systems is referred to as "static" because the excitation power source does not rotate, but is stationary or static. A compound excitation system of the static type is described, for example, in U.S. Pat. No. 3,702,965 issued Nov. 14, 1972 to Drexler et al. The patent to Drexler et al. describes an excitation system receiving energy from both potential windings and current windings. Hence, the term compound source is applied. The current windings provide electrical energy to the excitation system in response to the output current from the generator armature winding, that is, from the main machine output winding. The potential windings supply electrical energy to the excitation system in response to the voltage across the armature windings. Such compound source excitation systems exhibit response ratios of approximately 3.5 or better. The potential windings for such machines are typically found within the stator slots, lying over, but insulated from, the main armature winding. On the other hand, the current windings for the excitation system generally surround each phase of the three typical phases of the armature output winding leads.
However, simpler excitation systems are possible, particularly where a high response ratio is not required. A simple excitation system, for example, appears to be disclosed in U.S. Pat. No. 3,132,296 issued May 5, 1964 to Nippes. However, the excitation system in this patent is solely directed toward utilizing the third and higher harmonics of the fundamental frequency of the rotor magnetic flux. Such systems as disclosed therein are not practical for large dynamoelectric machines. (As used herein, the term large dynamoelectric machine refers to one which has a power rating in excess of approximately 50 megawatts.) The third harmonic or other higher harmonic is not capable of providing sufficient power to the excitation system to produce such high levels of generator output. Unlike the cooling requirements of large machines, the cooling requirements for the excitation system components of a machine employing only third or higher harmonics of the rotor magnetic flux, are minimal. However, certain excitation system components of large machines require cooling to operate effectively and reliably over extended periods of time.